In the motor vehicle industry, the requirements made on car lacquering are altering in response to changed quality requirements. Requirements such as high gloss and mechanical stability, e.g. against gravel, have long been made. Other requirements were made later, with regard to resistance of the lacquered surfaces to petrol and oil. Environmental changes, however, result in additional stresses. One important aspect is "acid rain". The lacquer industry is therefore required to produce acid-resistant coatings on articles subjected to normal weathering. Other corrosive conditions can occur if animal excrement falls on the lacquer and remains there for a time.
Earlier test methods frequently used only dilute acetic acid to test the stability of the lacquer, but new test methods require much more severe conditions. For example, up to 40% sulphuric acid is used in tests, sometimes at elevated temperatures of up to 70.degree. C. The lacquer must show practically no change in protective properties and appearance at the places tested. At most, the surface can be slightly roughened by the acid attack.
Another aspect of concern to the lacquer industry is the handling of physiologically dangerous substances, e.g. carcinogens or mutagens. Isocyanates or aromatic solvents are examples of compounds which may be hazardous.
When lacquer binding agents based on lacquer systems cross-linked via free isocyanate groups are used, volatile reactive isocyanate may enter the gas phase during application and thus endanger the persons at work. Isocyanate-containing coating agents should also as far as possible be kept from contact with the skin.
For this reason, there has already been research on avoiding the use of reactive isocyanates in coating agents for the car industry. EP-A-34 720 describes oxazolidine-containing acrylate resins which cross-link with water or atmospheric humidity to form films. However, the system has the disadvantage that the hardening surface prevents water from penetrating more deeply into the lower layers of lacquer so that they can harden fully. Other binder systems, known e.g. from EP-A-50 249, comprise silicon-containing polymers which react with moisture and can cross-link via hydrolysis-sensitive groups on the Si atom. These systems have the disadvantage that cross-linking depends on the relative air humidity. On the other hand, during storage care must be taken that no .moisture reaches the coating agent, since otherwise normal processing may be impossible.
U.S. Pat. No. 4,408,018 describes acrylic polymers into which acetoacetate groupings have been introduced. These are designed to cross-link with .alpha., .beta.-unsaturated esters. The catalysts for this cross-linking reaction have to be strong bases. These remain in the coating film after hardening and adversely affect the properties of the coating, e.g. yellowing or turbidity of the clear lacquer film.
DE-OS 35 41 140, EP-A-160 824 and EP-A-310 011 describe hardening components and binder systems for the lacquer industry. The substances are solvent-containing two-component systems (2K systems) produced on the basis of a polyunsaturated acrylate resin and a CH-, NH- or SH-active hardening component. Catalytically active compounds in the lacquer system are necessary in order to obtain rapid and adequate cross-linking. The following examples of catalysts are mentioned: tertiary amines, amidines, guanidines, phosphanes and halides of quaternary ammonium compounds. The quantity of catalysts can be up to 5% by weight. They are needed for accelerating the hardening reaction, which may optionally be assisted by elevated temperatures.
The previously-mentioned coating-agents must be used in the form of two-component systems. They contain catalytically active compounds and therefore cannot be formulated as single-component systems (1K systems). Catalyst-containing 2K systems mixed before application have a pot life not exceeding 48 hours, after which the coating agent is no longer usable. If these known coating agents are used as clear lacquers or covering lacquers, they have reduced resistance to yellowing when exposed to light. In addition, the known coating agents do not meet the previously-mentioned more exacting criteria as regards corrosion by acid.